In a conventional diesel-electric locomotive, a main generator directly coupled to a diesel engine generates electric power and supplies the electric power to a traction motor via an electric power converter. The diesel engine and the main generator generate heat; therefore, they are cooled down by an engine fan mechanically direct-coupled to the engine. Furthermore, the traction motor and the electric power converter also generate heat; therefore, they are cooled down by either the engine fan or an electric fan mechanically direct-coupled to an auxiliary generator.
The engine fan rotates at the same rotation speed as the engine; therefore, the air flow volume is approximately proportional to the engine rotation speed. In addition, an AC generator, which excels at maintenance, is generally used as the auxiliary generator, and the output frequency thereof is proportional to the engine rotation speed. An induction motor, which excels at maintenance, is generally used as the electric fan, and the rotation speed thereof is approximately proportional to the frequency of its power supply, so that the air flow volume is approximately proportional to the engine rotation speed.
When the locomotive accelerates, the traction motor is supplied with output power from the diesel engine, through the main generator and the electric power converter; therefore, the quantity of heat generated by each of those devices is proportional to output power from the diesel engine. For this reason, the rotation speed of the diesel engine is made approximately proportional to its output power, so that the air flow volumes made by the engine fan and the electric fan are approximately proportional to the rotation speed of the diesel engine. Operating the diesel engine with its rotation speed being approximately proportional to its output power results in high fuel efficiency, thereby leading to a system configuration that gives high efficiency during accelerations.
On the other hand, when the locomotive decelerates, the traction motor and the electric power converter convert into electric power kinetic energy of the locomotive and its cars, so that a braking resistor dissipates this electric power. During this period, the engine produces little mechanical power but the traction motor and the electric power converter generate heat; therefore, it is necessary to cool them down by the engine fan or the electric fan. Each of the fans rotates approximately proportionally to the engine rotation speed and the air flow volume thereby is proportional to its rotation speed; therefore, in order to adequately cool them down, it is necessary to rotate the engine at a higher speed.
When the diesel-electric locomotive runs into a tunnel and such, efficiency of cooling thereof is worsened; therefore, the speed of the train has been controlled in such a way that the quantity of the heat generated is within a range of its cooling capability.
Patent Document 1: Japanese Patent Laid-Open No.2000-203420